Group III-N compounds, such as gallium nitride (GaN) and its related alloys have been under intense research in recent years due to their promising applications in electronic and optoelectronic devices. Particular examples of potential optoelectronic devices include blue light emitting and laser diodes, and UV photodetectors. Their large bandgap and high electron saturation velocity also make them excellent candidates for applications in high temperature and high-speed power electronics.
Due to the high equilibrium pressure of nitrogen at growth temperatures, it is extremely difficult to obtain GaN bulk crystals. Owing to the lack of feasible bulk growth methods, GaN is commonly deposited epitaxially on substrates such as SiC and sapphire (Al2O3). However, a current problem with the manufacture of GaN thin films is that there is no readily available suitable substrate material which exhibits close lattice matching and close matching of thermal expansion coefficients.
SiC is an excellent thermal conductor, but is very expensive and only available in small wafer sizes. Presently, (0001) oriented sapphire is the most frequently used substrate for GaN epitaxial growth due to its low price, availability of large-area wafers with good crystallinity and stability at high temperatures. However, the lattice mismatch between GaN and sapphire is over 13%. Such huge mismatch in the lattice constants causes poor crystal quality if GaN films were to be grown directly on the sapphire, due to stress formation and a high density of defects, including such defects as microtwins, stacking faults and deep-levels. Typically, these GaN thin films exhibit wide X-ray rocking curve, rough surface morphology, high intrinsic electron concentration and significant yellow luminescence.
The most highly refined semiconductor substrate in the world are silicon wafers. Silicon is increasingly being used as a substrate for GaN materials. Silicon substrates have been considered for use as substrates for growth of GaN films. Silicon substrates for GaN growth is attractive given its low cost, large diameter, high crystal and surface quality, controllable electrical conductivity, and high thermal conductivity. The use of Si wafers promises easy integration of GaN based optoelectronic devices with Si based electronic devices.
The disadvantages of Si as a substrate for GaN heteroepitaxy include a +20.5% a-plane misfit which initially led to the conclusion that growth of GaN directly on silicon was not likely to work well. In addition, the thermal expansion misfit between GaN (5.6×10−6K−1) and Si (6.2×10−6K−1) of 9.6% can lead to cracking upon cooling for films grown at high temperature. Thus, direct growth of GaN on substrates including Si has been found to result in either polycrystalline growth, substantial diffusion of Si into the GaN film and/or a relatively high dislocation density (e.g. 1010 cm−2). Moreover, GaN is also known to poorly nucleate on Si substrates, leading to an island-like GaN structure and poor surface morphology. Thus, the quality of GaN films grown on silicon has been far inferior to that of films grown on other commonly used substrates such as sapphire or silicon carbide. Moreover, the growth conditions that have been used for GaN on Si are generally not compatible with standard silicon processing.
Numerous different buffer layers have been disclosed for insertion between the substrate and the GaN layer to relieve lattice strain and thus improve GaN crystal quality. Thin AlN, GaAs, AlAs, SiC, SiO2, Si3N4 and ZnO or low-temperature GaN layers have been used as a buffer layers for GaN growth on Si. However, even when buffer layers are used, the thermal expansion co-efficient mismatch is typically too large to suppress the formation of cracks in the GaN and other related Group III-N films grown on silicon.